Negative pressure assisted tissue treatment system

ABSTRACT

A system for stimulating the healing of tissue comprises a porous pad positioned within a wound cavity, and an airtight dressing secured over the pad, so as to provide an airtight seal to the wound cavity. A proximal end of a conduit is connectable to the dressing. A distal end of the conduit is connectable to a negative pressure source, which may be an electric pump housed within a portable housing, or wall suction. A canister is positioned along the conduit to retain exudates suctioned from the wound site during the application of negative pressure. A controller for automated oscillation of pressure over time is provided to further enhance and stimulate the healing of an open wound. A power management motor control for varying pump drive frequency and managing a portable power supply is provided to increase battery life and improve patient mobility.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation of U.S. patent application Ser. No.14/282,208, filed May 20, 2014, which is a continuation of U.S. patentapplication Ser. No. 12/862,885, filed Aug. 25, 2010, which is acontinuation of U.S. patent application Ser. No. 11/364,264, filed Feb.28, 2006, now U.S. Pat. No. 7,811,269 which is a continuation of U.S.patent application Ser. No. 09/939,166 filed Aug. 24, 2001, now U.S.Pat. No. 7,004,915, which claims priority to International ApplicationNo. PCT/US2002/027070, filed Aug. 23, 2002, which designated the UnitedStates, all of which are hereby incorporated by reference.

BACKGROUND OF THE INVENTION

Field of the Invention

This invention relates generally to tissue treatment systems. Moreparticularly this invention relates to vacuum assisted treatment systemsthat aid in the healing of open wounds.

Description of Related Art

Vacuum induced healing of open wounds has recently been popularized byKinetic Concepts, Inc. of San Antonio, Tex., by its commerciallyavailable V.A.C.® produce line. The vacuum induced healing process hasbeen described in commonly assigned U.S. Pat. No. 4,969,880 issued onNov. 13, 1990 to Zamierowski, as well as its continuations andcontinuations-in-part, U.S. Pat. No. 5,100,396, issued on Mar. 31, 1992,U.S. Pat. No. 5,261,893, issued Nov. 16, 1993, and U.S. Pat. No.5,527,293, issued Jun. 18, 1996, the disclosures of which areincorporated herein by this reference. Further improvements andmodifications of the vacuum induced healing process are also describedin U.S. Pat. No. 6,071,267, issued on Jun. 6, 2000 to Zamierowski andU.S. Pat. Nos. 5,636,643 and 5,645,081 issued to Argenta et al. on Jun.10, 1997 and Jul. 8, 1997, respectively, the disclosures of which areincorporated by reference as through fully set forth herein. Additionalimprovements have also been described in U.S. Pat. No. 6,142,982, issuedon May 13, 1998 to Hunt, et al.

In practice, the application to a wound of negative gauge pressure,commercialized by Assignee or its parent under the designation “VacuumAssisted Closure” (or “V.A.C.®”) therapy, typically involves themechanical-like contraction of the wound with simultaneous removal ofexcess fluid. In this manner, V.A.C.® therapy augments the body'snatural inflammatory process while alleviating many of the knownintrinsic side effects, such as the production of edema caused byincreased blood flow absent the necessary vascular structure for propervenous return. As a result, V.A.C.® therapy has been highly successfulin the promotion of wound closure, healing many wounds previouslythrough largely untreatable.

The frequency at which negative pressure is applied to the wound, aswell as the frequency of the pressure change over time, ahs a directimpact on the rate of wound healing. A variation of pressure change overtime, not provided by current vacuum assisted therapy devices, isthought to significantly increase the rate of wound healing. Similarly,a rapid return to normal activities for the patient receiving woundtherapy, may also improve the rate of wound healing, as increasedphysical activity is often accompanied by increased vascularcirculation, which in turn leads to improved blood flow at the woundsite. One barrier to a return to normal activities is limited batterylife, which is a result of the electrical power required to powerexisting vacuum assisted wound therapy systems. Additionally, frequentinspection of the wound site is required in order to ensure the wound isnot becoming infected. However, a rapid return to normal activities mustnot preclude the precautions that must be utilized during use of vacuumassisted therapy to prevent inadvertent spillage of wound exudates fromthe canister, or entry of the wound exudates into the pumping mechanism.

Additional limitations are associated with the use of fixed frequencyoscillating pumps in the prior art. Such limitations are the result ofthe size of the pump required to maintain the desired negative pressureat the wound site, and/or a reduction in battery life due to the powerrequired to operate the oscillating pumps. Oscillating pumps, as knownin the art, are typically designed for limiting operating conditions.For example, to maximize low pressure flow rate at a fixed frequency.Typically the mass and/or stiffness of various components are altered tochange the resonant frequency of the pump under the design operatingconditions. If the pressure across the pump increases, the stiffness ofthe system is increased by back pressure across the diaphragm of theoscillating pump. The resonant frequency of the pump changes and thefixed frequency drive is not driving the pump at the optimum frequency.As a result, flow rate drops quickly and the capability of the pump todrive air at high pressure is limited. Accordingly, in order to provideincreased flow rate at higher pressures requires either a sacrifice inflow rate at low pressures, or a pump of significantly greater size,when utilizing a fixed frequency oscillating pump.

For the foregoing reasons, there is a need for a vacuum assisted woundtreatment system that is capable of automated pressure change over time.Additionally, there is a need for a more efficient vacuum assisted woundtreatment system, that allows the patient more mobility, while reducingthe risk of exudate spillage or pump contamination.

It is therefore an object of the present invention to provide a vacuumassisted wound treatment system that provides a means for increasing thestimulation of cellular growth by a variation of pressure over time.

A further object is to provide a system that is capable of extendedoperation in the absence of an alternate current power supply.

An additional object of the present invention is to provide a sanitaryand cost effective means for sampling fluids drawn from the wound sitewithout necessitating removal of the canister, or disturbing of thewound site.

Still another object of the present invention is to provide a vacuumassisted wound therapy device that can be secured to an object so as toreduce the likelihood of disturbance to the device, while still allowingconvenient placement for its operation.

BRIEF SUMMARY OF THE INVENTION

In accordance with the foregoing objects, the present inventiongenerally comprises a porous pad for insertion substantially into awound site and a wound drape for airtight sealing enclosure of the padat the wound site. A distal end of the a tube is connected to thedressing in order to provide negative pressure at the wound site. Afluid sampling port is provided on the tube to allow for sampling ofwound fluids being drawn through the tube from the wound site. A sourceof negative pressure is in communication with a proximal end of thetube. A collection canister is removably connected to the tube forcollection of fluid removed from the wound during the application ofnegative pressure. A first filter is incorporated into an opening of thecanister, and a second filter is positioned between the canister and thesource of negative pressure. As the source of negative pressure may bean electric pump, supplied by alternating or direct current, a powermanagement device, and its associated power management protocol, isincorporated to maximize battery life when the unit is being supplied bydirect current. A clamping mechanism is utilized to secure the system toa stationary object, such as a bed rail, or pole, such as that used tosuspend a container of intravenous fluid.

The pad, comprised of a foam having relatively few open cells in contactwith the areas upon which cell growth is to be encouraged so as to avoidunwanted adhesions, but having sufficiently numerous open cells so thatdrainage and negative pressure therapy may continue unimpaired, isplaced in fluid communication with a vacuum source for promotion offluid drainage, as known in the art. The porous pad of the presentinvention may be comprised of polyvinyl alcohol foam. The fluidcommunication may be established by connecting a tube to a dressing,such as that described in International Application WO 99/13792,entitled “Surgical Drape and Suction Heads for Wound Treatment,” thedisclosure of which is incorporated herein.

Upon placement of the pad, an airtight seal is formed over the woundsite to prevent vacuum leakage. Such a seal may be provided by placing adrape over the wound, such that the drape adheres to the healthy skinsurrounding the wound site, while maintaining an airtight seal over thewound itself.

A conduit or tube is placed in fluid communication with the foam pad,its distal end communicating with a fluid drainage canister which is influid communication with a vacuum source. A constant or intermittentnegative pressure therapy is conducted as described in the prior art.Alternatively, the negative pressure is varied over time, so as tofurther stimulate cell growth, which in turn may shorten the healingprocess. The negative pressure induced on the wound adjusts to meet avarying target pressure, which oscillates between a target maximum andtarget minimum pressure.

Flow rate of a variable displacement pump, used in accordance with thepresent invention, is maximized over a pressure range by varying thedrive frequency of the pump. The optimum drive frequency is continuouslyadjusted by a system that periodically or continuously monitors thepressure across the pump to determine the optimum drive frequency forthat pressure. Pump performance is thereby improved over variabledisplacement pumps utilized in the prior art, without increasing pumpsize or weight. Similarly, pump performance of a typical variabledisplacement pump can be achieved with a smaller pump, which in turnreduces the size and weight of the overall system in order to improveease of use and portability for the patient. An alternative negativepressure source, such as a fixed displacement pump, sometimes referredto as a positive displacement pump, may also be utilized.

The power management system is utilized to maximize battery life whenthe present invention is being supplied with electric power under directcurrent. The power management system comprises deactivation of abacklight to a display terminal, or touch screen liquid crystal display(LCD) control panel, after a predetermined interval. Battery life isfurther extended when the power management system prevents electricpower from reaching an electric motor until the targeted power settingis actually large enough to activate the motor. In such an instance, themotor is utilized to provide negative pressure by driving an electricpump as known in the art.

The foregoing has outlined some of the more pertinent objects of thepresent invention. These objects should be construed to be merelyillustrative of some of the more prominent features and applications ofthe invention. Many other beneficial results can be attained by applyingthe disclosed invention in a different manner or by modifying theinvention as will be described. Accordingly, other objects and a fullerunderstanding of the invention may be had by referring to the followingDetailed Description of the Invention, which includes the preferredembodiment.

BRIEF DESCRIPTION OF THE DRAWINGS

These and other features and advantages of the invention will now bedescribed with reference to the drawings of certain preferredembodiments, which are intended to illustrate and not to limit theinvention, and wherein like reference numbers refer to like components,and in which:

FIG. 1 is a schematic block diagram of a tissue treatment systemutilized in accordance with the present invention.

FIG. 2A is a perspective view of a fluid sampling port utilized inaccordance with the present invention.

FIG. 2B is a perspective view of an alternative embodiment of a fluidsampling port utilized in accordance with the present invention.

FIG. 3A is a perspective view of the back portion of a pump housingutilized in accordance with the present invention.

FIG. 3B is a perspective view of the front portion of a pump housingutilized in accordance with the present invention.

FIGS. 4A and 4B are flow charts representing the preferred steps in theimplementation of a power management system utilized in accordance withthe present invention.

FIG. 5 is a flow chart illustrating the preferred steps in theimplementation of pulse therapy utilized in accordance with the presentinvention.

DETAILED DESCRIPTION OF THE ILLUSTRATIVE EMBODIMENTS

Although those of ordinary skill in the art will readily recognize manyalternative embodiments, especially in light of the illustrationsprovided herein, this detailed description is exemplary of the preferredembodiment of the present invention, the scope of which is limited onlyby the claims that are drawn hereto.

The present invention is a vacuum assisted system for stimulating thehealing of tissue.

Referring now to FIG. 1 in particular, there is illustrated the primarycomponents of a system that operates in accordance with the presentinvention. The present invention 10 includes a foam pad 11 for insertionsubstantially into a wound site 12 and a wound drape 13 for sealingenclosure of the foam pad 11 at the wound site 12. The foam pad 11 maybe comprised of a polyvinyl alcohol (PVA) open cell polymer material, orother similar material having a pore size sufficient to facilitate woundhealing. A pore density of greater than 38 pores per linear inch ispreferable. A pore density of between 40 and 45 pores per linear inch ismost preferable. Such a pore density translates to a pore size ofapproximately 400 microns.

Addition of an indicating agent, such as crystal violet, methylene blue,or similar agents known in the art causes a color change in the foam 11when in the presence of a bacterial agent. As such, a user or healthcare provider can easily and readily ascertain if an infection ispresent at the wound site 12. It is contemplated that the indicatingagent may also be placed in line of the conduit 16, between the woundsite 12 and the canister 18. In such a configuration (not shown), thepresence of bacterial contaminants in the wound site 12, could be easilyand readily ascertained without disturbing the wound bed, as there wouldbe a nearly immediate color change as bacterially infected woundexudates are drawn from the wound site 12 and through the conduit 16during application of negative pressure.

It is also contemplated that the foam pad 11 may be coated with abacteriostatic agent. Addition of such an agent, would serve to limit orreduce the bacterial density present at the wound site 12. The agent maybe coated or bonded to the foam pad 11 prior to insertion in the woundsite, such as during a sterile packaging process. Alternatively, theagent may be injected into the foam pad 11 after insertion in the woundsite 12.

After insertion into the wound site 12 and sealing with the wound drape13, the foam pad 11 is placed in fluid communication with a vacuumsource 14 for promotion of fluid drainage and wound healing, as known tothose of ordinary skill in the art. The vacuum source 14 may be aportable electrically powered pump, or wall suction as commonly providedin medical care facilities.

According to the preferred embodiment of the present invention, the foampad 11, wound drape 13, and vacuum source 14 are implemented as known inthe prior art, with the exception of those modifications detailedfurther herein.

The foam pad 11 preferably comprises a highly reticulated, open-cellpolyurethane or polyether foam for effective permeability of woundfluids while under suction. The pad 11 is preferably placed in fluidcommunication, via a plastic or like material conduit 16, with acanister 18 and a vacuum source 14. A first hydrophobic membrane filter20 is interposed between the canister 18 and the vacuum source 14, inorder to prevent wound exudates from contaminating the vacuum source 14.The first filter 20 may also serve as a fill-sensor for canister 18. Asfluid contacts the first filter 20, a signal is sent to the vacuumsource 14, causing it to shut down. The wound drape 13 preferablycomprises an elastomeric material at least peripherally covered with apressure sensitive adhesive for sealing application over the wound site12, such that a vacuum seal is maintained over the wound site 12. Theconduit 16 may be placed in fluidic communication with the foam 11 bymeans of an appendage 17 that can be adhered to the drape 13.

According to the preferred method of the present invention, a secondhydrophobic filter 22 is interposed between the first filter 20 and thevacuum source 14. The addition of the second filter 22 is advantageouswhen the first filter 20 is also used as a fill sensor for the canister18. In such a situation, the first filter 20 may act as a fill sensor,while the second filter 22 further inhibits contamination of woundexudates into the vacuum source 14. This separation of functions in asafety device and a control (or limiting) device, allows for each deviceto be independently engineered. An odor vapor filter 23, which may be acharcoal filter, may be interposed between the first filter 20 and thesecond filter 22, in order to counteract the production of malodorousvapors present in the wound exudates. In an alternate embodiment (notshown), the odor vapor filter 23 may be interposed between the secondhydrophobic filter 23 and the vacuum source 14. A second odor filter 15may be interposed between the vacuum source 14 and an external exhaustport 25, in order to further reduce the escape of malodorous vapors fromthe present system. A further embodiment allows for first 20 and secondfilters 22 to be incorporated as an integral part of the canister 18 toensure that the filters 20, 22, at least one of which are likely tobecome contaminated during normal use, are automatically disposed of inorder to reduce the exposure of the system to any contaminants that maybe trapped by the filters 20 and 22.

A means for sampling fluids may also be utilized by providing aresealable access port 24 from the conduit 16. The port 24 is positionedbetween the distal end 16 a of the conduit 16 and the proximal end 16 bof the conduit 16. The port 24, as further detailed in FIGS. 2A and 2B,is utilized to allow for sampling of fluids being suctioned from thewound site 12. Although the port 24 is shown as an appendage protrudingfrom the conduit 16, it is to be understood that a flush mounted port(not shown) will serve an equivalent purpose. The port 24 includes aresealable membrane 26 that after being punctured, such as by ahypodermic needle, the seal is maintained. Various rubber-like materialsknown in the art for maintaining a seal after puncture can be utilized.

The process by which wound fluids are sampled, utilizing the presentinvention, comprises penetrating the membrane 26 with a fluid sampler28, such as a hypodermic needle or syringe. The sampler 28 is insertedthrough the membrane 26 and into the port 24 until it is in contact withwound fluids flowing through the inner lumen 30 of the conduit 16. Asillustrated in FIG. 2B, and further described in U.S. Pat. No.6,142,982, issued to Hunt, et al. on May 13, 1998, and whose referenceis incorporated herein as though fully set forth, the inner lumen 30 maybe surrounded by one or more outer lumens 31. The outer lumens 31 mayserve as pressure detection conduits for sensing variations in pressureat the wound site 12. In an alternative embodiment (not shown), theouter lumen or lumens 31 may act as the negative pressure conduit, whilethe inner lumen 30 may act as the pressure detection conduit. In thepresent invention, the fluid sampling port 24, communicates only withthe inner lumen 30, so as not to interfere with pressure detection thatmay be conducted by the outer lumens 31. In an alternate embodiment (notshown) in which the outer lumen 31 serves as the negative pressureconduit, the fluid sampling port 24 communicates with the outer lumen31.

The vacuum source 14 may consist of a portable pump housed within ahousing 32, as illustrated in FIGS. 3A and 3B. A handle 33 may be formedor attached to the housing 32 to allow a user to easily grasp and movethe housing 32.

According to the preferred embodiment of the present invention, a meansfor securing the housing 32 to a stationary object, such as anintravenous fluid support pole for example, is provided in the form aclamp 34. The clamp 34, which may be a G-clamp as known in the art, isretractable, such that when not in use is in a stored position within arecess 36 of the housing 32. A hinging mechanism 38 is provided to allowthe clamp 34 to extend outward from the housing 32, to up to a 90 degreeangle from its stored position. An alternative embodiment (not shown)allows the clamp 34 to be positioned at up to a 180 degree angle fromits stored position. The hinging mechanism 38 is such that when theclamp 34 is fully extended, it is locked in position, such that thehousing 32 is suspended by the clamp 34. A securing device 40, such as athreaded bolt, penetrates through an aperture 42 of the clamp 34, toallow the clamp 34 to be adjustably secured to various stationaryobjects of varying thickness.

Alternatively, the securing device 40, may be comprised of a springactuated bolt or pin, that is capable of automatically adjusting tovarious objects, such as intravenous fluid support poles, having varyingcross-sectional thickness.

The present invention also allows for management of a power supply tothe vacuum source 14, in order to maximize battery life when the presentinvention is utilizing a direct current as its power supply. In thepreferred embodiment, as illustrated in the flow chart of FIG. 4A, amotor control 44 determines if the actual pressure is less than or equalto a target pressure 46. If the actual pressure is less than the targetpressure, a tentative motor drive power required to reach the targetpressure is calculated 48. If the tentative motor drive power requiredto reach the target pressure is greater or equal to the stall power 49,the tentative motor drive power is actually applied to the motor 50. Ifthe actual pressure is greater than the target pressure, the tentativemotor drive power is decreased and a determination is made as to whetheradditional power is needed to overcome the stall power 52. If it isdetermined that the tentative power is inadequate to overcome the stallpower, the tentative power is not supplied to the motor 54. If thetentative power is adequate to overcome the stall power, the tentativepower is actually applied to the motor 50. The motor control 44functions as a closed loop system, such that the actual pressure iscontinuously measured against the predetermined target pressure. Theadvantage of such a system is that it prevents power from being suppliedto the motor when it is not necessary to maintain the target pressurespecified for V.A.C.® therapy. Accordingly, battery life is extendedbecause power is not needlessly used to power the motor when it is notnecessary.

Battery life is further extended, as illustrated in the flow chart shownin FIG. 4B, by providing a means, such as an integrated software programin a computer processor, for automatically disengaging a backlight ofthe visual display 19 of the present invention 10 (as seen in FIG. 3B).User input of information 55, such as target pressure desired, orduration of therapy, activates 57 a backlight of the visual display 19shown in FIG. 3B. User input 55 may also be simply touching the visualdisplay 19, which may be a touch activated or a pressure sensitivescreen as known in the art. Activation of an alarm 55 may also activate57 the backlight of the display 19. An alarm may be automaticallyactivated if an air leak is detected at the wound site 12. Such a leakmay be indicated by a drop of reduction in pressure being detected atthe wound site 12. The backlight remains active until a determination ismade as to whether a preset time interval has elapsed 58. If the timeinterval has not elapsed, the backlight remains active 57. If the timeinterval has elapsed, the backlight is automatically extinguished 59,until such time as the user inputs additional information, or an alarmis sounded 55.

Referring now back to FIG. 1, battery life is further extended by meansof a variable frequency pump drive system 80, when the pump 14, used inaccordance with the present invention, is an oscillating pump. The pumpdrive system 80 consists of a pressure sensor 82, a control system 84,and a variable frequency drive circuit 86. In the preferred embodimentthe pressure sensor 82 measures the pressure across the pump, which isrelayed to the control system 84. The control system 84 determines theoptimum drive frequency for the pump 14 given the pressure measured andrelayed by the pressure sensor 82. The optimum drive frequency for thepump 14 may be determined by the control system 84 either repeatedly orcontinuously. The control system 84 adjusts the variable frequency drivecircuit 86 to drive the pump at the optimum frequency determined by thecontrol system 84.

The use of the variable frequency pump drive system 80 allows thepressure of the pump 14 to be maximized. In tests on sample oscillatingpumps, the maximum pressure achieved was doubled by varying the drivefrequency by only 30%. Additionally, the system 80 maximizes flow rateover the extended frequency range. As a result, performance of the pump14 is significantly improved over existing fixed frequency drive systempumps without increasing the pump size or weight. Consequently, batterylife is further extended, thus giving the user greater mobility by nothaving to be tethered to a stationary power source. Alternatively, asimilar performance level to the prior art fixed frequency drive systempumps can be achieved with a smaller pump. As a result, patient mobilityis improved by improving the portability of the unit.

The preferred embodiment also increases the stimulation of cellulargrowth by oscillating the pressure over time, as illustrated in the flowchart of FIG. 5. Such as oscillation of pressure is accomplished througha series of algorithms of a software program, utilized in conjunctionwith a computer processing unit for controlling the function of thevacuum source or pump. The program is initialized when a user, such as ahealth care provider, activates the pulsing mode of the pump 60. Theuser then sets a target pressure maximum peak value and a targetpressure minimum peak value 62. The software then initializes thepressure direction to “increasing” 63. The software then enters asoftware control loop. In this control loop, the software firstdetermines if the pressure is increasing 64.

If the actual pressure is increasing in test 64, a determination is thenmade as to whether a variable target pressure is still less than themaximum target pressure 70. If the variable target pressure is stillless than the maximum target pressure the software next determineswhether the actual pressure has equaled (risen to) the ascending targetpressure 66. If the actual pressure has attained the ascending targetpressure, the software increments the variable target pressure by oneinterval 68. Otherwise, it refrains from doing so until the actualpressure has equaled the ascending target pressure. If the variabletarget pressure has reached the maximum target pressure in the testblock 70 the software sets the pressure direction to “decreasing” 69 andthe variable target pressure beings to move into the downward part ofits oscillatory cycle.

The interval may be measured in mmHg or any other common unit ofpressure measurement. The magnitude of the interval is preferably in therange of about 1 to 10 mmHg, according to the preference of the user.

If the actual pressure is decreasing in test 64, a determination is thenmade as to whether the variable target pressure is still greater thanthe minimum target pressure 74. If the variable target pressure is stillgreater than the minimum target pressure the software next determineswhether the actual pressure has attained (fallen to) to the descendingtarget pressure 76. If the actual target pressure has equaled thedescending target pressure the software decrements the variable targetpressure by one interval 72. Otherwise it refrains from doing so untilthe actual pressure has equaled the descending target pressure. If thevariable target pressure has reached the minimum target pressure in thetest of block 74, the software sets the pressure direction to“increasing” 73 and the variable target pressure beings to move into theupward part of its oscillatory cycle. This oscillatory process continuesuntil the user de-selects the pulsing mode of the pump 61.

While the invention has been described herein with reference to certainpreferred embodiments, these embodiments have been presented by way ofexample only, and not to limit the scope of the invention. Accordingly,the scope of the invention should be identified only in accordance withthe claims that follow.

We claim:
 1. A system for stimulating healing of tissue at a wound siteto be covered by a drape for treatment, the system comprising: a pumpingmechanism adapted to deliver negative pressure to a wound cavity betweenthe drape and the wound site, the pumping mechanism including a pump andan electric motor having a stall power and coupled to the pump togenerate the negative pressure; a pressure sensor adapted to measure thenegative pressure and having an output to provide a signal indicative ofan actual pressure provided by the pump; and a power management motorcontrol adapted to compare the actual pressure to a target pressure andcalculate a tentative motor drive power that is applied to the electricmotor for reaching the target pressure when the tentative motor drivepower is greater than or equal to the stall power, and prevented frombeing applied to the electric motor when the tentative motor drive poweris less than the stall power.
 2. The system of claim 1, furthercomprising a control system to determine an optimum drive frequency forvariably driving the pump in order to maximize pump flow.
 3. The systemof claim 2, further comprising a variable frequency drive circuit fordriving the electric motor at said optimum drive frequency.
 4. Thesystem of claim 1, further comprising a drainage tube having a distalend and a proximal end, the distal end of the drainage tube beingfluidly connected to the wound cavity and the proximal end being fluidlyconnected to the pump.
 5. The system of claim 1, further comprising aportable power unit for supplying power to the electric motor.
 6. Thesystem of claim 1, further comprising a computer processing unit adaptedto vary the negative pressure by comparing the actual pressure to amaximum target pressure and a minimum target pressure.
 7. The system ofclaim 1, wherein said electric motor is a variable frequency motor. 8.The system of claim 7, further comprising a control system to determinean optimum drive frequency for driving the variable frequency motor inorder to maximize pump flow.
 9. The system of claim 8, furthercomprising a variable frequency drive circuit for driving the variablefrequency motor at said optimum drive frequency.
 10. The system of claim1, further comprising a control system for varying the negative pressureover a time interval by comparing the actual pressure to a varyingtarget pressure that is incremented between a maximum target pressureand minimum target pressure and adjusting the actual pressure to meetthe varying target pressure in response to the comparisons.